fix(FilePreview): set fixed size to image container

[Antreesy]

☑️ Resolves

• Minor UX changes to FilePreview
  • fix container size to single source (prevents jumping to 0 and back when image is loaded)
  • use NcLoadingIcon
  • don't load virtual image in mounted hook
  • add fade transition from blurhash/loader to image

🖌️ UI Checklist

🖼️ Screenshots / Screencasts

🏚️ Before

before.mp4

🏡 After

after.mp4

🏁 Checklist

• 🌏 Tested with different browsers / clients:
  • Chromium (Chrome / Edge / Opera / Brave)
  • Firefox
  • Safari
  • Talk Desktop
  • Not risky to browser differences / client
• 🖌️ Design was reviewed, approved or inspired by the design team
• ⛑️ Tests are included or not possible
• 📗 User documentation in https://github.com/nextcloud/documentation/tree/master/user_manual/talk has been updated or is not required

[ShGKme]

Didn't test, looks good on screencast

[nickvergessen]

/backport to stable30

[Antreesy]

Didn't test

Maybe we should 🙈

• image container size is fixed to small square when loading:
  

• and when upload is failed: